Boston Marathon Eyewitness Gives FBI Photo of Man Fleeing Scene

A series of photos showing a man in burned and shredded clothing fleeing from the site of the Boston Marathon finish line Monday is one of the more than 2,000 images the FBI is analyzing as it searches for clues.

The photos, taken by Boston businessman Ben Thornike from his office building nearby, show a man in tattered, seemingly burned clothes sprinting from the scene immediately after the two blasts that killed three people and injured more than 170 people erupted.

"I was struck by this individual," Thornike told ABC News. "Everyone else in the photo is on the ground, bent over, holding their ears, appearing to be in shock, immobile, but this one individual is exactly the opposite."

"He's sprinting. His clothes were in tatters," he said. "They were melted off, burned off his body, so for all I know he was panicked and fleeing."

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Authorities have not said that man - or any other individual - is a suspect in the case, but they have contacted Thornike about the photos and have repeatedly urged any witnesses to come forward with videos and photos that could be a potential piece of evidence.

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That plea for help, including sending authorities to Boston's Logan Airport to catch departing travelers, has resulted in the more than 2,000 images the FBI, along with a team of analysts dispatched to Boston from the Marine Corps Base in Quantico, Va., is now analyzing in painstaking frame-by-frame comparisons.

Authorities are also currently looking for information on someone who was carrying an unusually heavy dark backpack in the area before the race.

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An unclassified security bulletin sent by the FBI Tuesday to law enforcement agencies across the country showed images of a shredded backpack found at the marathon explosion site in which one of the mangled, partially exploded "pressure cooker" bombs had reportedly been hidden.

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Adding to the potential evidence in this case, Boston news station WHDH has also come forward with footage that shows a large sack in front of a metal fence just seconds before one of the bombs detonated, after which the large bag disappeared.

"Assistance from the public remains critical in establishing a timeline of events which leads to swift conclusions through due diligence and strong investigative activity," FBI Special Agent in charge Richard DesLauriers said Tuesday.

The FBI urges anyone with potential information to call 800-CALL-FBI.

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ABC News' John Santucci, Rhonda Schwartz, Brian Ross and Pierre Thomas contributed to this report.